扬子台地西北缘志留纪笔石Oktavitesspiralis(Geinitz,1842)发育过程研究

THE ASTOGENIES OF SILURIAN GRAPTOLITE OKTAVITES SPIRALIS BASED ON SPECIMENS FROM THE NORTHWESTERN MARGIN OF THE YANGTZE PLATFORM,SOUTH CHINA

Oktavites spiralis(Geinitz,1842)是志留系特列奇阶(Telychian)的笔石带化石,其形态特征明显,在全球范围内广泛分布,地层对比意义较强。然而,该种宽泛的化石定义以及缺乏居群变异研究,造成了对该化石延限的争议,不利于地层的精确对比。笔者对产自陕西紫阳、岚皋地区特列奇阶Oktavites spiralis带至Cyrtograptus lapworthi笔石带的45个Oktavites spiralis不同发育阶段的幼体标本进行研究,建立了较完整的发育序列,从而对该种的始部特征有了更为深入的了解,增加了描述的精确性。同时,通过始部特征的对比研究,可将Oktavites spiralis和与之共生的Oktavites contortus(Perner,1897)明确区分为2个独立的种。此外,对Oktavites spiralis和Oktavites contortus发育过程的研究,也可成为该属进一步精确厘定的主要依据之一。

A graptolite index species of Telychian （late Llandovery, Silurian）, Oktavites spiralis Geinitz, 1842, is significant in stratigraphic correlation due to its typical morphological characters, rapid evolution and global distribution. However, controversies on the stratigraphic range of the species derived from its broad definition without information of population variation has led to the difficulties in regional correlation. In order to increase the accuracy of description, 45 specimens of Oktavites spiralis with different developmental stages have been collected from the Oktavites spiralis Zone to the Cyrtograptus lapworthi Zone （Telychian） in the Ziyang-Langao area, and its developmental sequence has been established according to the larvas which the sicula and proximal rhabdosomes are also well preserved. Meanwhile, Oktavites spiralis and its coexisting species Oktavites contortus can distinguished according the different of the proximal rhabdosomes, which to define this genus precisely. be further morphology is also a key